Photocatalytic process for purifying water polluted by tetrahydrothiophene

ABSTRACT

Process for the photocatalytic purification of waste water contaminated by tetrahydrothiophene, applicable to the purification of condensation water collected along gas distribution lines for household and industrial use.

The present invention relates to a perfected process for thephotocatalytic purification of water contaminated bytetrahydrothiophene.

Tetrahydrothiophene, an odorizing thioether, is normally added to gasfor household and industrial use to reveal accidental losses along thedistribution lines. Collection siphons of condensation water whichcontains a quantity of a few parts per million of the odorizing compound(THT), are installed along the gas transportation network.

These siphons are periodically emptied and the water recovered with atypically strong smell, due to the presence of the odorizing agentcondensed together with it, making it necessary to carry out anappropriate flushing treatment before discharging into the atmosphere.

Among possible kinds of treatment, adsorption on active carbon has thedisadvantage of also generating another waste product, consisting of thecarbon itself impregnated with a badly-smelling material which mustconsequently be incinerated, together with other typical disadvantageslinked to the treatment of liquid streams on active carbon beds.

Stripping with air is difficult to apply as the gaseous stream, ifdischarged into the air, causes obvious drawbacks. It is thereforenecessary to collect the gaseous stream and effect further treatment toblock the odorizing agent.

Treatment with chemical oxidants, such as hypochlorite for example, onthe one hand eliminates the odor of the water but does not remove thepresence of polluting compounds due to the oxidation of the originalodorizing agent, normally characterized by a poor biodegradability.

A technique which is very frequently proposed for degrading watercontaminated by organic compounds, consists in the photodegradation ofthe contaminants by means of radiation, solar or U.V., operating in thepresence of metal oxide catalysts of the semiconductor type. Thetreatment, which takes place in the presence of oxygen, generally atroom temperature, leads to the "mineralization" of the contaminants,which means the complete degradation of the toxic pollutant, with theformation of simple substances such as water, carbon dioxide and mineralacids.

Semiconductor metal oxides, such as titanium dioxide for example, arematerials which have a particular electronic configuration: the valenceband, at a lower energetic level, is completely occupied by theelectrons, whereas the conduction band, with a higher energy, is almostcompletely empty.

When a similar semiconductor is radiated with energy photons higher thanthe "bandgap" (interval between the two bands), or equivalently withlight having a wave-length lower than the "bandgap", the electrons passfrom the valence band to the conduction band, leaving an electronicdeficiency in the valence band and consequently causing the formation ofelectron-hole couples. The electrons are transferred to thesemiconductor/liquid interface and the interactions between theelectrons and/or the holes photogenerated, and the species adsorbed onthe surface of the semiconductor and oxygen present in the system, causethe degradation of the contaminants.

From a technical-environmental point of view, photocatalysis has thefollowing advantages with respect to the traditional technologies:

complete mineralization of a wide range of organic contaminants;

high separation efficiency at concentrations of the organic contaminantof the order of ppb;

absence of regenerative processes of the thermal type (as required forexample by active carbon) which often create problems of anenvironmental nature relating to gaseous emissions; and

the non-creation of microbic fouling phenomena typical of carbonfilters.

We have now solved these problems with a photocatalytic process whichenables the degradation of tetrahydrothiophene to obtain, in a simpleand inexpensive way, an odorless water without pollutants or itsintermediate degradation products.

In accordance with this, the present invention relates to a process forthe purification of condensation water collected along gas distributionlines for household and industrial use, contaminated bytetrahydrothiophene, effected with the following steps:

dispersing solid particles of semiconductor metal oxide, havingdimensions of about 0.5-3 μm, in said condensation water, in such aquantity that the concentration of the metal oxide is between 20 and1000 ppm;

supplying oxygen or air to this dispersion at a pressure ranging from 50to 300 mm of Hg;

radiating this dispersion, at room temperature, with U.V. light between150 and 420 nm for a time ranging from 20 to 150 minutes.

According to the present invention, a semiconductor metal oxide in theform of dispersed particles is added, as catalyst, to the condensationwater. Catalysts which can be used for the purpose are ZnO, SnO₂, Fe₂O₃, TiO₂ and Fe₃ O₄. Among these titanium dioxide is preferred inanatase crystalline form for reasons of photocatalytic activity and itsstability under the operating conditions. The catalyst is convenientlyin the form of particles of 0.5-3μm, so as to form a good dispersion ofthe catalyst in the aqueous phase. The concentration of the catalystgenerally varies from 20 to 1000 ppm and is preferably about 50 ppm.

The dispersion of the catalyst in the polluted water is subjected toradiation with ultraviolet light until the organic contaminant containedtherein has been completely degraded.

The radiation capable of activating a semiconductor catalyst, especiallytitanium dioxide, is ultraviolet radiation, especially near-ultravioletradiation (300-400 nm). The radiation sources normally used aretherefore (high, medium or low pressure) mercury arc lamps, or xenonlamps which emit with the U.V. field. These devices are available on themarket.

Elongated cylindrical photochemical reactors are conveniently used, inwhich a tubular UV lamp is inserted along the greater axis of thereactor in order to obtain the highest possible quantitative yield.According to one embodiment of the invention, the suspension iscontinuously circulated between a container tank and the photochemicalreactor.

In any case the operating temperature is room temperature, ortemperatures close to room temperature, supplying oxygen or air to thephotodegradation environment, in relation to the oxygen requirement ofthe decontamination system, for example up to a pressure for this waterof about 200 mm Hg. When operating in accordance with the presentinvention, the times for complete, or almost complete, degradation ofthe tetrahydrothiophene are about 20-150 minutes.

At the end of the process, the tetrahydrothiophene is simply andeconomically eliminated to obtain completely deodorized water withoutthe organic pollutant, completely transformed into carbon dioxide andsulfate ion.

The following examples provide a better illustration of the presentinvention.

EXAMPLE 1

400 ppm of titanium dioxide (Degussa P25) in anatase crystalline form,in the form of particles having a size of 0.5-3 μm, are added to asample of condensation water collected from specific flushing siphonsalong the methane distribution line and containing tetrahydrothiophene(THT). The dispersion is stirred in the photocatalytic reactorconsisting of a glass container having a volume of 400 ml in which amercury vapor, low pressure, UV lamp having a power of 125 W, isimmersed. The container is hermetically closed with the UV lamp off, andpressurized with oxygen at a pressure of 100 mm Hg. An initial sample ofthe dispersion is taken from a sampling and is analyzed with a gaschromatograph connected to a mass spectrometer. Further samples of thedispersion are taken after one and two hours of stirring of thedispersion at room temperature and at a constant pressure of 100 mm Hgof oxygen, with the UV lamp switched off. The analytical results do notshow any variation in the initial concentration of tetrahydrothiophenepresent in the water to be purified.

EXAMPLE 2

A second sample of water, identical to that used in example 1 butwithout the titanium dioxide, is charged into the photoreactor andilluminated with the UV lamp immersed therein. The dispersion isstirred, the temperature being maintained at values close to roomtemperature by water circulation in a cooling jacket constructed aroundthe UV lamp. Samples of the dispersion are taken at time zero (lamp off)and after 15, 30 and 60 minutes of radiation. Analysis by gaschromatography-mass spectrometry indicate, after 60 minutes, a reductionin the initial concentration of THT of about 20% and the formation ofintermediate compounds among which thiophene and thyram. The odor of theodorizing compound of the water discharged from the reactor at the endof the experiment remains unaltered.

EXAMPLE 3

400 ppm of titanium dioxide Degussa P25 is added to a third sample ofwater, identical to that used in the previous tests, and the mixture ischarged into the photoreactor described above. The dispersionpressurized with oxygen at 100 mm Hg, as in the previous examples, isstirred at room temperature. After removing an initial sample, the UVlamp is switched on, the temperature of the dispersion being maintainedat values close to room temperature by means of water cooling, as in theprevious examples. Further dispersion samples are taken after 15, 30 and60 minutes and gas mass analyzed under the same conditions as theprevious examples. After only 15 minutes of radiation there is no signof THT or the intermediate products observed in example 2, of whichthere are not even noticeable traces considering the extreme sensitivityof the analytical instrument. The sample at 15 minutes no longer has thetypical odor of thioether.

EXAMPLE 4

Example 3 is repeated, using a smaller quantity of titanium dioxide,equal to 50 instead of 400 ppm. After 15 minutes of radiation theconcentration of THT is equal to about a tenth of the initialconcentration and after 30 minutes there are no traces of the startingtetrahydrothiophene or intermediate products observed in test 2.

We claim:
 1. A process for the purification of condensation watercollected along gas distribution lines for household and industrial use,contaminated by tetrahydrothiophene, effected with the followingsteps:dispersing solid particles of semiconductor metal oxide, havingdimensions of about 0.5-3 μm, in said condensation water, in such aquantity that the concentration of the metal oxide is between 20 and1000 ppm; supplying oxygen or air to this dispersion at a pressureranging from 50 to 300 mm of Hg; radiating this dispersion, at roomtemperature, with U.V. light between 150 and 420 nm for a time rangingfrom 20 to 150 minutes.
 2. The process according to claim 1,characterized in that the semiconductor oxide is titanium dioxide. 3.The process according to claim 1, characterized in that thesemiconductor oxide is dispersed in a quantity of 50 ppm.
 4. The processaccording to claim 1, characterized in that the ultraviolet radiation isbetween 300 and 400 nm.